The use of semiconductor diodes for voltage limitation is conventional. Such a conventional semiconductor diode is shown in FIG. 1. This semiconductor diode is a pn diode, a p-doped layer 2 having been diffused into a homogeneously n-doped region 1. In order to reduce the bulk resistance and to improve the ohmic connection of the n-type semiconductor to the metal plating, n-doped region 1is highly n-doped from the back of the wafer as an n+ doping (region 3). Reference numerals 4 and 5 denote metal layers. If a reverse voltage US is applied to such a diode, the current increases strongly as soon as breakdown voltage UZ is exceeded. The cause of the current increase and accordingly of the voltage limitation is based on the onset of the avalanche effect. When a reverse voltage is applied, a space charge region forms at the pn boundary surface. From a specific electrical field strength Ecrit of approximately 2·105 through 4·105 V/cm, charge carriers in the space charge region are accelerated so strongly that when they strike the crystal lattice, they break open bonds of the semiconductor and thus produce additional electrons and holes, which for their part are accelerated and are able to break open additional bonds. As a result, the current increases steeply.
A disadvantage of such diodes is that the breakdown voltage increases as the temperature rises. It possesses a positive temperature coefficient TK. If such a diode is briefly operated at high currents in breakdown, the power occurring as heat, the product of breakdown current IZ and breakdown voltage UZ may be very high. In diodes used in motor vehicle generators for rectification, powers in the range of several kilowatts occur during load-dump operation. As a consequence, the diode heats up and breakdown voltage UZ rises strongly. This is disadvantageous for many applications. For example, the maximum acceptable voltage in future 42-V electrical systems is only 58 V. Because the generator voltage in such systems may rise to 48 V, the temperature coefficient of the breakdown voltage must be negligibly small. In this connection, it should be noted that in addition to the influence of temperature on the breakdown voltage, it is also necessary to consider the bulk resistance and manufacturing tolerances.